Many treatments exist to improve the gait and function of persons with Cerebral Palsy (CP). Despite recognizing that muscle weakness is a major impairment in CP, none of the treatments have the direct aim of strengthening muscles. Our results from an NIH investigation (R01-NS035830) indicated high correlations between ankle strength and function, with greater strength correlated with higher function. The idea of strengthening muscles has been controversial for safety issues. The purposes of this pilot investigation are to: 1) establish sample sizes for a future randomized clinical trial determining if intensive ankle strength training programs can improve strength, gait, and function without increasing spasticity; and 2) investigate potential safety issues arising from the training programs. Aim 1: Establish sample sizes for a future randomized clinical trial determining if intensive ankle strength training programs can improve ankle strength, gait, and function without increasing spasticity. Twenty ambulatory subjects with spastic diplegia CP will be randomly assigned to one of 4 groups: 1) Dorsi-flexor strength training group; 2) Plantar-flexor strength training group; 3) Dorsi-plantarflexor strength training group; and 4) the group undergoing no intensive strength training program. Subjects in the strength training groups will participate in a 12-week progressive, resistance strength-training program. Prior to, and at the end of the training program, all subjects will be objectively assessed for ankle Plantar-flexor spasticity, ankle strength, gait, and GMFM. The data will be used in a repeated measures power analysis to establish sample sizes for the clinical trial. Aim 2: Investigate potential safety issues arising from the training programs. The aim has 2 components. The first is the weekly measurement of Plantar-flexor spasticity and tightness. The data will permit continuous monitoring of spasticity and tightness, and alert investigators to potential problems during each subject's participation. In the second component, both the pre- and post-intervention measures, and the weekly monitoring of spasticity and tightness, will determine if potential changes could be a concern for the future clinical trial.